With the development of a hybrid fiber coax (HFC), an x digital subscriber line (xDSL), and a fiber to the home (FTTH) during recent several years, high transmission speed has been accomplished in network environments. A broadband convergence network (BcN) has widened network bandwidths in Korea and next-generation broadband networks have been increasingly built all over the world. In addition, various wireless environments such as the 802.11 standard, satellite broadcasting, digital multimedia broadcasting (DMB), and wireless broadband Internet (WiBro) have been researched and developed according to various user environments. Accordingly, at present, small terminals having mobility and high-performance terminals such as existing personal computers (PCs) and televisions (TVs) coexist.
Video on demand (VOD) services provide videos in real time at user's requests. When using a VOD service, a user can see a drama that has already been broadcasted on the Internet or can see a movie on the Internet without renting a video from a video rental shop. In a real video on demand (RVOD) service, connection is newly made for each request of a user. A network bandwidth is limited and is not sufficient to provide a popular video for many users through RVOD. In other words, with only broadband networks, a video cannot be appropriately provided for hundreds of thousands of people or several million people through the RVOD at a time. To overcome this limit of the RVOD service, a near video on demand (NVOD) has been suggested. Conventional NVOD services will be briefly described below.
<Staggered Broadcasting>
RVOD requires a huge bandwidth of a server according to user's requests. To overcome this problem, various NVOD service methods have been suggested. Of these methods, staggered broadcasting is the most widespread conventional NVOD service method. In the staggered broadcasting, a whole video is transmitted through many channels at predetermined intervals so that many subscribers can view the video at a time. For example, when a 120-minute video is transmitted through 12 channels, a time interval between the channels is 10 minutes, and therefore, an access latency is 10 minutes. The access latency is a delay time from a request by a user (or a viewer) for a video to a play of the video. The staggered broadcasting is advantageous in that storage space is not necessary in a terminal (hereinafter, referred to as a client) and is disadvantageous in that the access latency is too great for the bandwidth of a server.
<Pyramid Broadcasting (PB)>
This method is the first method of dividing video data into segments and transmitting the segments. In the PB, video data is divided into many segments and the segments are transmitted using many channels. When a video is divided into “K” segments, “K” channels are needed. Here, the size of a segment is αi, where i=0, 1, 2, 3, . . . , k) and α≧1. When α is 2, data is divided into segments at a ratio of 1:2:4:8:16:32 . . . and the segments are repeatedly transmitted through the “k” channels. A viewer receives the video through the “k” channels which change in order. For example, when a 120-minute video is transmitted through 12 b channels (where “b” is a playback bit rate of the video), an access latency is reduced to 1 minute or less, which is at least 90% less than that in the staggered broadcasting. In other words, when the access latency is the same between the staggered broadcasting and the PB, the PB requires just a very small band compared to the staggered broadcasting. However, the PB is disadvantageous in that a client needs storage space for storing at least 50% of the video and also needs a bandwidth of 2 αb.
<Skyscraper Broadcasting (SB)>
SB is characterized by that it can be used in an environment with a client having limited storage space. Like in the PB, video data is divided into segments and the segments are transmitted through many channels in the SB. However, in the SB, the video data is divided at a ratio of 1:2:2:5:5:12:12:25:25:52:52 . . . , and therefore, the bandwidth efficiency of a server is increased. In addition, a client needs a network bandwidth of only 2 b. However, the SB has low channel efficiency and the channel efficiency is decreased when the network bandwidth of the client is increased.
<Harmonic Broadcasting (HB)>
In the HB, video data is uniformly divided into segments and all segments are simultaneously downloaded. Since it would be good only if each segment is downloaded by the time when it is supposed to be played, an i-th segment is transmitted with a bandwidth of
      1    i    ⁢      b    .  The HB provides good efficiency for a server but cannot be used for a client with a limited bandwidth since the client must receive data of all channels at a time. Moreover, when the length of a video increases, the number of logic channels greatly increases, and therefore, the complexity of a system also becomes huge.<Fast Broadcasting (FB)>
In the FB, video data is divided into segments having the same size and 2n-1 segments are transmitted through each channel. For example, the first segment is transmitted through the first channel and next two segments are transmitted through the second channel periodically. A client receives data from all channels. The amount of data is greatly increased when the channel changes sequentially, and therefore, the FB is possible with a small number of channels as compared to the above-described broadcasting services. However, the FB can be used only when a client is not limited in storage space and a network bandwidth. Accordingly, the FB is not suitable for various client environments.
<Fibonacci Pyramid Broadcasting (FPB)>
Like in the SB, a client has a network bandwidth of 2 b in FPB. However, the FPB provides a better dividing scheme than the SB by dividing video data at a ratio of a Fibonacci sequence like 1:2:3:5:13:21 . . . . Since numbers in the Fibonacci sequence increase more rapidly in those in the division ratio used in the SB, a video can be provided with a less number of channels in the FPB than in the SB. However, in the FPB, a transmission period increases by 2n-1 each time when transmission is repeated in a channel, and after three transmissions, the transmission period of the channel is greater than the size of data, which inhibits seamless playout of video.
The above-described conventional NVOD service methods require a large storage space for clients or a great network bandwidth for servers. It is difficult to use these conventional NVOD service methods for compact clients having a small storage space. Therefore, an NVOD service method that can be used in a network like a BcN including various types of clients is desired.